Modern electronic devices are typically formed by soldering electronic components together. Such devices can include logic or memory chips on printed circuit boards, multichip modules or complex integrated circuits. In these devices, numerous electrical connections must be formed between a component and its mating substrate (circuit board).
A more recent approach to joining electronic components is “surface mounting” in which metal regions or tabs on the components are aligned and soldered to corresponding metallized pads on the substrate. In these devices, numerous connections must be formed between the component and conductive circuitry on the substrate. The conducting paths may form a tight network or array of connecting points. Thus, accuracy and precision in placement of the conducting pads is essential to proper functioning of the device. In one common approach, often referred to as “ball grid array” BGA assembly, the metallized regions of the component or the mating pad of the substrate are prebumped with solder balls prior to assembly. The assembly is then heated so that reflow of the solder occurs, forming a permanent physical and electrical connection at each soldered point in the array.
However, the reflow of molten solder is not always reliable. The solder can spread further than desired causing short-circuits between adjacent conductive lines. In addition, the solder can migrate from the desired location creating a solder starved joint or one in which the electrical connection has failed entirely. As the density of interconnective lines becomes greater, the problem of solder reliability becomes more difficult to resolve.
Many modern electronic solder connections are thin, high aspect ratio, joints. The solder joint is plastically constrained and can develop triaxial (hydrostatic) stresses several times greater than the average tensile strength of the bulk solder material. In addition, stresses arise from the thermal cycling of electronic circuits as the device incorporating such electronics is turned on and off. The solder joint experiences the full shear resulting from changes in component dimensions with temperature. Thus, even when solder joints are initially satisfactory, the nature of the joint itself becomes a critical point for device failure over time.
There exists a need for better methods and compositions for joining electronic components which can reduce the likelihood of conductive joint failure during the initial soldering process, and/or inhibit subsequent joint fatigue and/or fracture. Methods and compositions that can improve the efficiency of assembling ball grid arrays (BGAs) and the like would provide a solution to a problem in the art of automated manufacturing of complex electronic devices. Likewise, soldering methods and compositions which reduce triaxial stresses and, hence, the potential for joint fatigue, solder fracture or other electronic failure of thin, high aspect, solder joints are needed in the arts.
The microelectronic industry relies on solder alloys with precisely controlled and reproducible properties. These properties include the temperatures of phase transitions, both melting upon heating and freezing upon cooling, which are important for the processes of package fabrication. The final, solid properties, such as electrical conductivity, mechanical strength and ductiility, grain structure and orientation are important to the operation and the longevity of solder joints.
Assembly of complex microelectronic packages require many different solder types. Two properties of growing concern are the undercooling and the crystal grain orientation. Undercooling (UC) or the degree of undercooling can be defined as the temperature difference between the solder's melting and freezing points. At the melting point, the solder composition would be almost entirely in a liquid state. At the freezing point, the solder composition would be almost entirely in a solid state. In between, there is a supercooling condition.
Most materials have small undercooling temperatures (UCT). But some solders, (notably tin containing solders such as SnCu compositions) have significant undercooling temperatures. Sometimes, these undercooling temperatures are on the order of 100 degrees C. This phenomenon arises because the crystallization of the solder composition requires, at the atom level, assembly from randomized atoms in the liquid state into highly ordered crystalline lattices. Sometimes the liquid form is in a pseudo-ordered state which must be overcome through a higher energy intermediate transition state in order to establish a different ordering on the atomic level to form the crystalline structure. Such is the case with tin which is an icosohedron phase structure in the melt (liquid state) and must first transition to a crystalline tetragonal structure of beta tin. This exacerbates the degree of undercooling for this solder composition.
In addition, the increased use of low alpha emitting solders, which are of higher purity, also exacerbates undercooling behaviors. In less pure solders, unintended impurities sometimes nucleate crystallization. Crystallization proceeds very rapidly once nucleation has taken place. Nucleation is favored when there are solid surfaces which can template the transition to the crystalline form. This is a type of solid phase heterotaxy where the final crystal structure maps off the exposed face crystal structure of the template. Other times the way in which the solid phase template acts is less certain as there is no known structural, dimensional, or other similarities which can be observed.
However fortuitous, this impurity-mediated nucleating effect cannot be relied upon. Therefore, substitutes must be found which give a measure of control during manufacture to the undercooling characteristics of the solder while not deleteriously affecting other properties of the solder joint itself, which the microelectronics industry has come to rely upon.